1. Field of the Invention
The present invention involves apparatus and methods for electrostatically spray coating substrates, particularly cylindrical substrates, to provide layers having uniform thickness along their entire lengths.
Electrophotographic imaging systems include a photo-receptor material which is electrically charged, exposed to light and then toner developed to form an image on the photoreceptor. This image is then transferred, either directly or indirectly onto a recording medium, i.e., paper, and fixed thereto. The photoreceptor material can be provided on a cylindrical substrate (a drum), in the form of a belt or in the form of a continuous web. Even when provided in the form of a belt, it is common to form photoreceptor layers on these belts by either forming a belt or placing a preformed belt on a cylindrical substrate and then coating the belt with the photoreceptor material. The coated belt is then removed from the substrate. See, for example, U.S. Pat. No. 4,747,992 to Sypula et al, the disclosure of which is herein incorporated by reference.
A spray coating process can be used for applying the photoreceptor material to a substrate (this applies whether a coated drum or a belt is ultimately formed). This spray coating process involves traversing a spray gun parallel to the longitudinal axis of a rotating cylindrical substrate and directing an atomized stream of photoreceptor material onto the substrate. Since the substrate is rotated while spraying takes place, the entire surface of the cylindrical substrate is coated. With proper controls the process can coat layer thicknesses from less than 100 Angstrom to more than 100 microns with better than .+-.5% reproducibility.
A major drawback of the above-described simple spray process is the relatively low efficiency with which material is applied. For some photoreceptor materials, only about 10% of the sprayed material coats the substrate. The excess sprayed material is carried past the drum and is captured by filters at the spray booth air exit. This low efficiency results in greatly increased coating solvent emissions which can necessitate the installation of solvent recovery equipment, further raising costs.
An established technique for the improvement of material efficiency is the application of an electrostatic charge to the sprayed fluid droplets. When the substrate is grounded, a positive attraction is created between the droplets and the substrate which causes the materials efficiency to increase to greater than 75%. Unfortunately, the use of electrostatic charge has a disadvantage in that cylindrical substrates coated by this method have coating thicknesses which vary along the length of the cylinder. Generally, thickness decreases toward the ends of the substrate. This thickness variation results from at least two sources: variations in the electrostatic forces between the substrate and droplets due to electrostatic "end effects" at the ends of the cylindrical substrate and attraction between the droplets and other nearby grounded surfaces such as the substrate support ("ground effects"). Since the cylindrical substrates are frequently supported on vertical supports which extend from a chain conveyor, these ground effects can be substantial.
Non-uniformity of the thickness of the photoreceptor material poses a substantial limitation to the use of electrostatic spraying processes for forming photoreceptor drums or belts. In order to produce uniform images, the photoreceptor material thickness must be uniform. Uniformity can be achieved by reducing the spray coating efficiency along thicker areas so as to produce a thickness equal to that of the thinnest areas. Uniformity can also be achieved by not using the end portions of the drum or belt. Neither of these alternatives is desirable since the first decreases the materials efficiency and the second alternative requires longer drums to be constructed which increases the size of the overall device. While it has been suggested that the electrostatic spray gun voltage be varied to compensate for thickness variation, the increase in charge on the droplets not only increases the attractive forces between the droplets and substrate, but also increase the attractive forces between the droplets and the other nearby grounded surfaces.